1. Field of the Invention
The present invention relates to an ultrashort pulsed laser device used in an optical communication field and an optical information processing field.
2. Related Background Art
Three-dimensional bit-by-bit recording, which is a technology expected as next generation high-density and high-capacity recording, exploits a non-linear effect such as a multiphoton process. When ultrashort pulsed laser with a pulse width in femtoseconds is collected onto a glass, an organic material and the like that are transparent with respect to laser light applied in a normal state using a lens, a transition probability of the absorption changes in proportion to the square of a light intensity, thus generating optical absorption. Thereby, the absorption spectrum can be changed by, for example, changing diarylethene from an open ring to a closed ring. Since the material is transparent with respect to laser light, it can be made a multilayer along a laminated direction, whereby optical recording can be increased in capacity.
Such three-dimensional bit-by-bit recording requires an ultrashort pulsed laser. FIG. 9 shows an example of a femtosecond laser as an ultrashort pulsed laser light source. In this device, laser light from a pump laser 60 is collected onto a solid laser medium 63 via a mirror 61 and a concave mirror 62. Between a saturable absorber mirror 64 and an output mirror 65, a resonator is constituted. The saturable absorber mirror 64 is used for mode lock. A prism pair 66 and 67 is aimed for dispersion compensation. Cr: forsterite (pumping wavelength: 1.06 μm, oscillation wavelength: 1.3 μm) is used as the solid laser medium 63, and a Nd: YAG laser (wavelength: 1.06 μm) is used as the pump laser 60, whereby ultrashort pulsed light of 20 fs is obtained.
Assuming that an average power, a repetition frequency, a pulse width and a peak power of this ultrashort pulsed light are Pc, f, t and Pp, respectively, energy E of one pulse is represented as follows:E=Pc/f(J).
And, the peak power Pp is represented as follows:Pp=E/t(W).
Pc depends on the power of the pump laser. To accommodate household electronic appliances, the pump laser has to be not more than 1 W. Therefore, in order to increase Pp, the repetition frequency f and the pulsed width should be decreased. Meanwhile, in order to apply the ultrashort pulsed laser to optical recording, for example, a considerable degree of data transfer rate is required, so that a repetition frequency of not less than several hundred MHz is required. In order to realize the peak power of 1 kW by 1-watt pumping, the pulse width needs to be made to be about 10 picoseconds.
As one way for decreasing the pulse width, the mode lock is preferable. According to this method, the phase of the pulse is made uniform by changing gain and loss of the resonator in synchronization with a timing of one reciprocating motion of the laser light through the resonator. Methods for the mode lock include active-type mode lock utilizing an AO element and an EO element and passive-type mode lock utilizing a saturable absorber and a Kerr effect. A relationship between a frequency ν of the mode lock and a resonator length d is represented as follows:ν=c/2d 
where c denotes the speed of light. Therefore, in order to realize the mode lock at the period of 100 MHz, the resonator length has to be set at 1.5 m. For that reason, the optical system becomes complicated as shown in FIG. 9, which causes a problem of increasing the size of the device, as well as a problem with reliability in view of the displacement and contamination of optical components.
In addition, there is a known ultrashort pulsed laser device as described in JP H08(1996)-213680 A in which an optical fiber is interposed in an optical path of a resonator. The use of the optical fiber allows a decrease in the number of optical components and the number of portions to be adjusted optically. However, in terms of a stability of the power by the mode lock and a simplification of the configuration, a sufficient practical device has not been realized.